1. Field of the Invention
The present invention relates to an immersion exposure apparatus that exposes a substrate to light passing through liquid and a device manufacturing method using the immersion exposure apparatus.
2. Description of the Related Art
To manufacture micro-semiconductor devices, such as semiconductor memories and logic circuits, using a photolithographic technique, reduction projection exposure apparatuses have been used. Reduction projection exposure apparatuses project a circuit pattern formed on an original, such as a reticle, onto a substrate, such as a wafer, using a projection optical system so that the circuit pattern is transferred onto the substrate.
The minimum critical dimension (the resolution) that can be transferred by a reduction projection exposure apparatus is proportional to a wavelength of light used for exposure and is inversely proportional to the numerical aperture (NA) of the projection optical system. Therefore, as the wavelength decreases and as the NA increases, the resolution becomes higher. Accordingly, in recent years, in response to a demand for reduction in the size of a semiconductor device, the wavelength of exposure light has been decreased. Thus, to decrease the wavelength of ultraviolet light used for exposure, rather than using a KrF excimer laser (wavelength=about 248 nm), an ArF excimer laser (wavelength=about 193 nm) is used.
In such a trend, immersion exposure has garnered much attention, since immersion exposure can further increase the resolution when a light source such as an ArF excimer laser is used. The immersion exposure decreases the effective wavelength of the exposure light by filling liquid in a space between the final surface of the projection optical system and a wafer (i.e., liquid is used for a medium disposed between the projection optical system and the wafer). Thus, the NA of the projection optical system can be increased in practice, and the resolution can be increased. The NA of the projection optical system is expressed as: NA=n×sin θ where n is an index of refraction of a medium. By filling a medium having an index of refraction higher than that of air (n>1), the NA can be increased to n.
In addition, liquid having a higher index of refraction provides a higher resolution. Accordingly, in place of immersion exposure apparatuses using pure water (purified water), an immersion exposure apparatus using liquid having a high index of refraction has been developed (refer to, for example, Japanese Patent Laid-Open No. 2006-004964).
In some immersion exposure apparatuses using liquid having a high index of refraction, in order to decrease a variation in transmittance of the liquid having a high index of refraction, a technique for purging an area around a liquid film by inactive gas has been developed (refer to, for example, Japanese Patent Laid-Open No. 2006-173295).
A first problem concerning the immersion exposure apparatuses using liquid having a high index of refraction is that the liquid having a high index of refraction tends to absorb oxygen easily as compared with pure water. Accordingly, when the liquid having a high index of refraction is exposed to air, the transmittance of the liquid for light in an ultraviolet light range is significantly decreased. The decrease in the transmittance of the liquid increases the temperature of the liquid due to absorption of exposure light. Accordingly, the index of refraction of the liquid varies. The variation in the index of refraction deteriorates the exposure performance (the image formation performance). To maintain high exposure performance, the transmittance of the liquid needs to be precisely controlled. In addition, to maintain high exposure performance, it is desirable that the transmittance of the liquid maintains a constant value. When the transmittance of the liquid varies, aberration of the projection optical system needs to be controlled in accordance with the variation.
To prevent oxygen from entering the liquid having a high index of refraction, Japanese Patent Laid-Open No. 2006-173295 describes the technique for purging an area around a liquid film by inactive gas. However, in such a case, the structure of a chamber is complicated.
A second problem concerning the immersion exposure apparatuses using liquid having a high index of refraction is that, since the liquid having a high index of refraction has a surface tension smaller than that of pure water, it is difficult to increase contact angles with respect to the surface of a wafer and the surface of a top plate of a wafer stage. Accordingly, in a local fill method, when exposure is performed while the wafer is moved together with the top plate, part of the liquid easily remains on the wafer or the top plate. If part of the liquid remains on the wafer or the top plate, the liquid may evaporate, and therefore, fluctuations in the conditions of an exposure environment, such as changes in temperature, may occur. In addition, when the liquid returns to a liquid film, air bubbles and turbulence may disadvantageously occur in the liquid film.